The present invention relates generally to the separation of stable or radioactive isotopes. Specifically, the present invention relates to a method and apparatus for the selective excitation of atoms or of polyatomic molecules for laser isotope separation using adiabatic population inversion on narrow multi-photon resonances.
The use of material enriched in one or more specific isotopes has become increasingly important. Many aspects of medicine, nuclear energy production and other areas of high-technology have significantly increased the demand for large quantities of materials enriched in certain isotopes at a relatively low cost. Specifically, the separation of isotopic species of heavier or high atomic weight elements is considerably more difficult than the separation of isotopic species of lower atomic weight elements. Thus, the complexity and cost aspects of the isotope separation of high atomic weight elements is a major problem.
It is well known to separate the isotopes of high atomic weight elements by use of gaseous diffusion. Gaseous diffusion has been one of the most widely used methods of isotope separation for heavy elements. The isotopes of elements differ by virtue of their mass, and not appreciably because of their chemical properties. Therefore, gaseous diffusion separation employs processes in which force produce different responses for the isotopes on the basis of mass.
The most rudimentary type of gaseous diffusion separator consists of a container with a porous membrane separating the container into two halves. A gas containing the material to be separated is supplied on one side of the porous membrane at a slight pressure, and a lower pressure or vacuum is maintained on the other side of the membrane. The molecules associated with each isotope have different masses. The average energies of the molecules, regardless of mass, are approximately the same. However, the velocity of the lighter molecules is higher than that of the heavier molecules. Thus, the frequency of collison against the porous membrane will be higher for the lighter molecules than the heavier molecules. Consequently, a slightly higher proportion of the lighter molecules will pass through the porous membrane. After a period of time has elapsed, the material on the low pressure side of the porous membrane will have a different ratio of numbers of the respective isotopes than the original mixture had. It can be appreciated that the separation of isotopes by gaseous diffusion is a massive and expensive undertaking.
More recently, the separation of isotopes has encompassed a broad range of technology. Additional methods of separating isotopes include electromagnetic separation, centrifugal separation and separation by selective excitation.
The development of laser systems, tunable to very narrow frequencies over a wide range of the spectrum, have significantly enhanced the utilization of laser technology in the field of isotope separation. Using tunable laser systems, the spectral response of the interaction of light with matter allows the production of selected reactions than can change the composition and properties of the matter.
The selective excitation of a specific isotope by a beam of laser light is a most promising technique for initiating isotope separation. Specifically, an infrared laser beam has been used for isotope separation by multiphoton excitation and dissociation of polyatomic molecules, and a visible excitation beam has been used for isotope separation by sequential (step-wise) excitation and dissociation of atoms. The excitation beams of the prior art have been precisely tuned to a particular wavelength and have a line-width sufficiently narrow so as not to excite other isotopes at nearby ground state wavelengths. The excitation beam uses a specific frequency and sufficient power to cause the molecule or atom containing the desired isotope to be excited to a specific energy state or to an average energy that is substantially higher than the average energy of the molecules or atoms containing the non-selected isotope or isotopes.
Once selective excitation has been made to occur, numerous processes contribute to de-exciting the isotopic species. An important loss mechanism is the collisional energy transfer between molecules. Thus, it is highly desirable that the selectively excited species be transformed to a stable or metastable state. The selectivity of the process for the desired species can be maintained through photoionization or photodissociation of the excited species. However, the processes of photoionization or photodissociation may not be selective.
Isotope separation using laser technology provides the capability of producing relatively large quantities of materials enriched in one or more isotopes at a relatively low cost. Typically, in molecular laser isotope separation the raw material to be enriched is in a gas phase. The gas phase is adiabatically expanded through a nozzle to a reduced temperature and high flow rate. The gas is rapidly cooled. It has been found that rapid cooling can be initiated by a nozzle which provides a means of reducing the population of excited vibrational states and thereby suppressing spectral interference due to transitions that originate from excited states. The cooled gas is irradiated by an infrared laser to selectively excite a specific vibrational state of an isotopic molecule in the gas mixture. The gas, which now includes the excited isotopic molecular species, is again irradiated. The second irradiation increases the energy of the excited isotope to a level where it may photodecompose, photo-ionize or photo-dissociate in a manner allowing separation from molecules containing other isotopes.
In atomic vapor laser isotope separation, a vapor of the chemical element the isotopes of which are to be separated is irradiated with one or more laser beams tuned to the frequencies of transitions between specific atomic states, and from one or more discrete atomic states to the ionization continuum or to resonances lying above the ionization threshold. The ions of the isotopic atoms are then subjected to an electric field in order to drive said atoms to impact on a collector plate which forms or is adjacent to the anode of the system that produces the electric field.
The rationale behind the above discussed processes is to utilize accurately tuned energy in the infrared or visible regions of the spectrum. The tuned energy will selectively excite only one of the isotopic species. The tuned energy is inadequate to excite the isotopic compound or atom which is absorbing the energy sufficiently to produce dissociation or ionization. The final excitation provides sufficient energy to produce ionization or dissociation. However, the line-widths associated with the second excitation are larger than at lower powers or intensities which greatly increases the difficulty of achieving selective absorption. Thus, photodissociation or photoionization is readily produced but it is not likely to be selective. Therefore, one isotopic species is selectively excited by the infrared laser, then an additional amount of energy is provided which is absorbed by both species. The second excitation drives the excited isotopic compound past the dissociation or ionization threshold. Whereas, the isotopic compound or atom that remained in the ground state during the initial infrared or visible irradiation is not sufficiently excited by the second irradiation pulse to be dissociated or ionized even though it also absorbs laser energy from the second excitation beam.
Numerous problems have been associated with the separation of isotopes of heavy elements. In general, it is more difficult to separate isotopes of heavy elements than isotopes of light elements. The isotope shifts of infrared or optical absorption lines are much smaller for atoms or compounds of heavy elements. A very large number of vibration-rotation states closely spaced in frequency are typically associated with gaseous polyatomic compounds of heavy elements. Selective light absorption is further complicated for heavy elements because at temperatures at which their compounds are gaseous, a large number of vibration-rotation states are already excited. The prior art in laser isotope separation utilizes selective excitation of one-photon resonances, for which the spectral complexity due to thermal excitation of a large number of vibration-rotation states may result in overlap of the single-photon resonances of the isotopic species which it is desired to separate. The overlap of the single-photon resonances of the various isotopic species also may increase at higher laser intensities of power densities, making efficient isotopic separation impossible to achieve.
There is, thus, a need for a laser isotope separation method and apparatus which provides isotope separation that is not restricted to the selective excitation from a ground state to a first excited state, which, at the same time, provides easy use for a broad range of polyatomic molecules, which is readily utilized with similar prior available systems, and which provides exceedingly efficient selection of the excited species.
Recognizing the need for an improved method and apparatus for laser isotope separation, it is, therefore, a general object of the present invention to provide a novel laser isotope separation method and apparatus which minimizes or reduces the problems associated with the presently known technology.
It is therefore a feature of the present invention to provide a laser isotope separation method and apparatus for efficiently selecting the specific species to be excited.
It is a more particular feature of the present invention to provide a laser isotope separation method and apparatus using adiabatic population inversion on narrow multiphoton resonances in polyatomic molecules.
It is another feature of the present invention to provide a laser isotope separation method and apparatus using adiabatic population inversion on multiphoton resonances in atoms.
Another feature of the present invention is to provide a laser isotope separation method and apparatus that can raise the majority of molecules or atoms of a preselected species to an excited state.
Yet another feature of the present invention is to provide a laser isotope separation method and apparatus using infrared, visible or ultraviolet laser radiation with a wavelength, an intensity and a pulse shape that have been chosen to produce an adiabatic population inversion.
Yet another feature of the present invention is to provide a laser isotope separation method and apparatus that initiates population inversion by the excitation of a narrow multiphoton spectral line composed of overlapping multiphoton resonanced originating from the rotational energy levels of the vibrational ground state of a polyatomic molecule or of overlapping multiphoton resonances originating from different hyperfine levels of the ground state of an atom.
Yet still another feature of the present invention is to provide a laser isotope separation method and apparatus that utilizes a population inversion and the narrow, overlapping multiphoton resonances which permit the excitation of nearly all of a preselected species.
Still another feature of the present invention is to provide a laser isotope separation method and apparatus that permits much higher selectivity and more efficient utilization of the isotopic atoms or polyatomic molecules to be separated.
Yet still another feature of the present invention is to provide a laser isotope separation method and apparatus that reduces or eliminates the requirement for extreme spectral simplification through the excitation of narrow multiphoton resonances rather than single-photon resonances.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized by means of the combinations and steps particularly pointed out in the appended claims.